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Hydropneumatic Sus. Facts...

Discussion in 'Suspension, Brakes, Wheels & Tires' started by SiRalex16v, Feb 22, 2007.

  1. SiRalex16v

    SiRalex16v New Member

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    Jun 22, 2006
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    So... when i was a kid... My dad had a citroen CX .. looks like this :) (i grew up in france which would explain why my dad had one of these :) )


    i remember when i would get in the car .. it would sit really low.. but as soon as we got inside and start it up .. it would raise up by its self. I later when i got older learned that this is a type of Hydrolic suspension that came from the factory :) what a great design for the 80's ... thought i would share some info on this neat suspension set up. enjoy!

    Hydropneumatic is a type of automotive suspension system.

    The purpose of this system is to provide a soft, comfortable, yet well-controlled ride quality. Its nitrogen springing medium is approximately six times more flexible than conventional steel, so self-leveling is incorporated to allow the vehicle to cope with the extraordinary suppleness provided. France was noted for poor road quality in the post-war years, so the only way to maintain relatively high speed in a vehicle was if it could easily absorb road irregularities.

    It was invented by Citroën and fitted to Citroën cars, as well as being adapted by other car manufacturers, notably Rolls-Royce, Mercedes-Benz and Peugeot. It was also used on Berliet trucks. Similar systems are also used on some military vehicles.

    While the system has inherent advantages over steel springs, generally recognized in the auto industry, it also has an element of complexity, so automakers like Mercedes-Benz, British Leyland (Hydrolastic, Hydragas), and Lincoln have sought to create simpler variants.

    Over 9 million vehicles have been produced with this type of suspension.

    This system uses a belt or camshaft driven pump from the engine to pressurise a special hydraulic fluid, which then powers the brakes, suspension and power steering. It can also power any number of features such as the clutch, turning headlamps and even power windows. The suspension system usually features driver-variable ride height, to provide extra clearance in rough terrain.

    The suspension setup is referred to as 'oléopneumatique' in early literature, pointing to oil and air as its main components.

    Citroën quickly realized that standard brake fluid was not ideally suited to high pressure hydraulics. They invented a new, green fluid, LHM. LHM stands for Liquide Hydraulique Minéral and is a mineral oil, quite close to automatic transmission fluid. Mineral oil is not hygroscopic (ie will not absorb water from the air) unlike standard brake fluid, so therefore gas bubbles do not form in the system, as used to be the case with standard brake fluid, creating a 'sprongy' brake feel. Use of mineral oil has thus spread beyond Citroën, Rolls-Royce, Peugeot, and Mercedes-Benz, to include Jaguar, Audi, and BMW.

    There have been many improvements to this system over the years, including variable ride firmness (Hydractive) and active control of body roll (Activa). The latest incarnation features a simplified single pump-accumulator sphere combination.

    The system had one key negative impact on the inventor, Citroën - only specialist garages were qualified to work on the cars - making them seem radically different from ordinary cars with common mechanicals. Citroën thus encountered the same issue as Apple Computer did - building a proprietary system deprives you of positive network externalities.

    Auto manufacturers are still trying to catch up with the combination of features offered by this 1955 suspension system, typically by adding layers of complexity to an ordinary steel spring mechanical system.


    Citroën first introduced this system in 1954 on the rear suspension of the Traction Avant. The first full implementation was in the advanced DS in 1955.

    Major milestones of the hydropneumatics design were:

    * During World War II, Paul Magès, an employee of Citroën, with no formal training in engineering, secretly develops the concept of an air/oil suspension to combine a new level of softness with vehicle control and self-leveling
    * 1954 Traction Avant 15H: Rear suspension, using LHS hydraulic fluid.
    * 1955 DS: Suspension, power steering, brakes and gearbox/clutch assembly powered by high pressure hydraulic assistance. A belt driven 7-piston pump, similar in size to a power steering pump generates this pressure when the engine is running.
    * 1962 Morris introduces the BMC ADO16 {'1100'} with hydrolastic suspension
    * 1964 Mercedes-Benz introduces the 600 with air suspension designed to avoid Citroën patents
    * 1965 Rolls-Royce licenses Citroën technology for the suspension of the new Silver Shadow
    * 1966 Mercedes-Benz introduces the 6.3 also with air suspension
    * 1967 The superior LHM mineral fluid is introduced
    * 1970 GS: Adaptation of the hydropneumatic suspension to a small car
    * 1970 SM: Variable speed auto-returning power steering, dubbed DIRAVI, and hydraulically actuated directional high beams
    * 1974 The Mercedes-Benz 450SEL 6.9 becomes the first hydropneumatic Mercedes-Benz automobile, with the pump driven by the engine's timing chain instead of an external belt. This adaptation was used only for the suspension. Power steering and brakes were conventional hydraulic- and vacuum-powered, respectively.
    * 1983 Citroën BX, built as a 4WD in 1990
    * 1989 XM: electronic regulation of the hydropneumatic system; sensors measure acceleration and other factors
    * 1990 Peugeot 405 Mi16x4: first Peugeot equipped with rear hydropneumatic suspension
    * 1993 Xantia: Optional 'Activa' (active suspension) system, eliminating body roll by acting on torsion bars. An 'Activa' equipped Xantia was able to reach more than 1 g lateral acceleration
    * 2001 C5: No more central hydraulic pressure generation; combined pump/sphere unit for the suspension only and with electric height adjustment sensors
  2. SiRalex16v

    SiRalex16v New Member

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    Jun 22, 2006
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    At the heart of the system, acting as pressure sink as well as suspension elements, are the so called 'spheres', five or six in all; one per wheel and one main accumulator as well as a dedicated brake accumulator on some models. They consist of a hollow metal ball, open to the bottom, with a flexible desmopan rubber membrane, fixed at the 'equator' inside separating top and bottom. The top is filled with nitrogen at high pressure, up to 75 bar, the bottom connects to the car's LHM fluid circuit. (See hydraulic accumulator). The high pressure pump powered by the engine pressurizes the circuit and an accumulator sphere. This part of the circuit is between 150 and 180 bars. It powers the front brakes first, prioritised via a security valve, and depending on type, can power steering, clutch, gearchange etc.

    Pressure goes from this circuit to the wheel spheres, pressurizing the bottom part of the spheres and rods connected to the wheel suspension. Suspension works by the rod pushing LHM into the sphere, compacting the nitrogen in the upper part of the sphere, the damping is provided by a two-way 'leaf valve' in the opening of the sphere. LHM has to squeeze back and forth through this valve which causes resistance and controls the suspension movements, it is the simplest damper and one of the most efficient. Car height correcting works by height correctors connected to the anti-roll bar, front and rear. These height correctors allow for more fluid to travel under pressure to the rod/sphere system when detecting that the suspension is lower than its expected ride height (e.g. the car is loaded) and when the car is too high (e.g. after unloading). Height correctors act with some delay in order not to correct regular suspension movements. Rear brakes are powered from the rear suspension spheres. Because the pressure there is proportional to the load, so is the braking power.


    The whole high pressure part of the system is manufactured from steel tubing of small diameter, connected to valve control units by Lockheed type pipe unions with special seals made from desmopan rubber, a type of rubber compatible with the LHM fluid. The moving parts of the system e.g. suspension strut or steering ram are sealed by extremely small tolerances between the cylinder and piston for tightness under pressure. The other plastic/rubber parts are return tubes from valves such as the brake control or height corrector valves, also catching seeping fluid around the suspension push-rods. The metal and alloy parts of the system rarely fail even after excessively high mileages but the rubber components (especially those exposed to the air) can harden and leak, typical failure points for the system.

    Spheres are subject to no mechanical wear but suffer pressure loss, mostly from nitrogen naturally diffusing through the membrane. They typically keep between 60,000 and 100,000 km. Spheres originally used to have a valve on top and be rechargeable. Newer spheres do not have this valve anymore, but it can be retrofitted. Though a rechargeable sphere has a longer lifespan, the membranes will eventually wear out though this can take over 20 years. A ruptured membrane means suspension loss at the attached wheel, however ride height is unaffected. Or in the case of the accumulator sphere, reliance on the high pressure pump as the only source of braking pressure to the front wheels.


    * maintains ride height independent of load - Self-levelling is integral
    * constant ride height is critical to vehicle aerodynamics
    * excellent ride comfort - vehicle seems to be floating above the road surface
    * isolation from road lessens driver fatigue on long journeys
    * large loads make no real changes to the handling
    * compact suspension design
    * maintenance for trained mechanic is relatively easy,
    * inexpensive in mass production
    * damping is very well controlled - the suspension is very soft but never wallows
    * the driver feels in complete command of the vehicle - a unique combination of precision roadholding without the harshness of a "sports" car
    * For vehicles that would otherwise have a conventional power steering pump, hydropneumatic adds no new equipment and in many cases results in a lower unsprung mass
    * Ride height adjustment is easily incorporated - allows the vehicle extra clearance over rough terrain
    * compensates automatically to allow travel with flat tire
    * can be conveniently interconnected in the roll plane to improve roll stiffness and thus roll stability limit, especially for heavy vehicles.
    * can be connected in the pitch plane to improve braking dive and traction squat.
    * the pressure exerted between the tyres of the same axle upon body roll is not subject to the same differential as on some other cars, the pressure in one suspension strut e.g.left hand, equals the pressure in the right hand through Pascal's law giving the 'light' tyre potentially more footprint pressure.
    * If they are interconnected in the three-dimensional full car model, the interconnected hydro-pneumatic suspension could realize enhanced roll and pitch control during excitations arising from steering, braking/traction, road input and crosswind, as with the Hydractive arrangement
    * flexibility in the suspension strut design in the interconnected suspension system to realize desirable vertical, roll and pitch properties for different types of vehicles.
    * horizontal orientation of the rear suspension cylinders below the level of the boot floor means that the full width of the boot is available for loads.
    * Mechanical steel spring suspension systems that try to replicate some of the inherent advantages of hydropneumatic suspension (multilink, adjustable shock absorbers) end up more complex to build and maintain than the straightforward hydropneumatic layout
    * Hydropneumatic suspension scares potential buyers and dealers, for people prepared to carry out simple maintenance you can acquire a luxury car that is superior in many ways to its' peers for a fraction of the cost. This is only a disadvantage if you haven't done the maintenance work.
  3. SiRalex16v

    SiRalex16v New Member

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    Jun 22, 2006
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    * Service requires a trained mechanic - non-specialists are uncomfortable with system
    * Suspension spheres and hydraulic fluid need maintenance every two years
    * Poorly maintained vehicles can have expensive problems
    * Body roll can be noticeable in older versions of the system. In newer models barely any roll is experienced.
    * Floating sensation can lead to sea sickness in some passengers
    * Failure of the hydraulic system (due to leak or belt breakage) can prevent vehicle travel at normal speed, similar to loss of power steering belt or brake fluid in a conventional vehicle
    * Car lowers when system depressurizes - generally innocuous or inconvenient at worst except for the legal ban on this feature imposed in the US in 1974.
    * Although physically no more complex than a traditional gas strut and coil spring set up, Hydropneumatics are not as intuitive to understand. This leads to people feeling wary of the technology and in turn reluctant to purchase older cars (without warranties) that use it.


    Hydractive Suspension is a new automotive technology introduced by the French manufacturer Citroën in 1990. It describes a development of the 1955 Hydropneumatic suspension design using additionally electronic sensors and driver control of suspension performance. The driver can make the vehicle stiffen (sport mode) or ride in outstanding comfort (soft mode). Sensors in the steering, brakes, suspension, throttle pedal and gearbox to feed information on the car's speed, acceleration, and road conditions to on-board computers. Where appropriate - and within milliseconds - these computers switched an extra pair of suspension spheres in or out of circuit, to allow the car a smooth supple ride in normal circumstances, or greater roll resistance for better handling in corners. This development keeps Citroën in the forefront of suspension design, given the widespread goal in the auto industry of an Active Suspension system. All auto suspension is a compromise between comfort and handling. Auto manufacturers try to balance these aims and locate new technologies that offer more of both.

    Citroën hydractive suspension was available on several models, including the XM and Xantia, which had a more advanced sub-model known as the Activa.

    The 2003 Citroën C5 has continued development of Hydractive suspension. Compared to earlier cars, the C5 stays at normal ride height even when the engine is turned off for an extended period, through the use of electronics. The C5 also uses a new, incompatible orange fluid, rather than the familiar green LHM mineral oil used in millions of hydropneumatic vehicles.


    * The accumulator sphere typically has enough pressure left for multiple hard brakings (up to 50), even if the engine (and attached high pressure pump) stops working.
    * Hydropneumatic cars can drive on 3 wheels. One can remove one rear wheel, and the car will be able to compensate by pressurising the other 3 wheels higher.
    * Maseratis in the 1970s borrowed some parts of the system for braking and power steering, but the only Maserati that did use a hydropneumatic suspension was the Quattroporte II. Citroën owned Maserati at this time and produced the 'Citroën-Maserati' SM.
    * Rolls-Royce cars used the system in combination with a standard suspension.
    * The first hydropneumatic cars used a different liquid called LHS (liquide hydraulique synthétique). This was thought to have superior properties over LHM, but it turned out to eat the rubber of the return tubes when aging and absorb atmospheric moisture turning acidic, thoroughly immobilising thousands of 50s cars. This probably contributed to uneasiness of the systems reliability.
    * Hydropneumatic parts are finely machined to the micrometre level of tolerance - a level not otherwise seen in auto manufacture
    * The "poor man's" alternative to hydropneumatic is British Leyland's Hydragas system, which uses pre-pressurised spheres which are interconnected between axles, and various air suspension arrangements
  4. SiRalex16v

    SiRalex16v New Member

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    Jun 22, 2006
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    Bump this up .. its a good learning piece for you guys so you can learn sus. history.. :)

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